The invention is directed to a reciprocating pump for a use as a medication administering device and has a piston which is displaceable with an actuation means. An admission fluid wing encompasses an admission line and an admission chamber, and has a valve part moveable in the admission chamber which is provided for closing an admission opening at the admission line.
For reasons of miniaturization, it is advantageous to utilize a miniaturized reciprocating pump as a pump unit in an implantable medication administering device. The piston displacement of such a pump needed in this application lies on the order of magnitude of 1 .mu.l. The pump should also be capable of pumping gas bubbles with a satisfactory conveying rate so that, given the appearance of a gas bubble in the fluid wing, the patient need not do without the medication . For example, a gas bubble in the medication reservoir arises due to the degasification of the medication or when replenishing medications. The problem of gas bubble conveying is raised to an even greater degree in medication dosing devices wherein the medication reservoir is charged with a low pressure. Such a low pressure in the medication reservoir is advantageous for safety-related reasons and typically fluctuates between 0.5 and 1 bar absolute. Given an ideal pump having an admission chamber free of dead space, a reduction of the gas conveying rate at the lower limit of the low pressure (0.5 bar absolute) thereby derives by the factor two in comparison to the fluid conveying rate for physical reasons. When a pump has a dead space in an admission chamber having the same size as the piston displacement, then physical reasons make a conveying of gas bubbles impossible at the lower limit of the low pressure (0.5 bar absolute) in the reservoir system. A gas bubble situated in the admission chamber is merely compressed and decompressed without having a conveying event toward the exit side occuring.
It is desirable for these reasons to have the greatest possible ratio of piston displacement to dead space in the admission chamber in a reciprocating pump. Since small quantities of fluid are to be conveyed in a medication administering device, an enlargement of the piston displacement of the reciprocating pump is usually not possible. The point of attack for achieving a high gas conveying rate thus lies in reducing the dead space.
A reciprocating pump of the species initially cited is disclosed by U.S. Pat. No. 3,568,250, particularly FIGS. 2 and 3. This provides an admission chamber which is formed by the end face of the piston, by a cylindrical wall and by an end face of a housing having an opening for an admission line. A moveable valve part is arranged inside the admission chamber. A coil spring holds the valve part pressed against the admission opening in a quiescent condition. A relatively large dead space arises due to the coil spring with the appertaining fastening means, this dead space potentially having an unfavorable effect on the gas conveying rate of the reciprocating pump.
German published application No. 19 11 649, FIGS. 1 through 4, shows an over-flow valve which is preferably utilized as a safety and control valve in blood circulations. The valve has a valve part having a magnetic closing force which can be adjusted from the outside. Here, however, the valve part and the valve seat are fashioned in a flow-beneficial way. That means that no sharp transmissions or salient mounts are present, this leading to considerable dead space. Further, a leak flow is present according to FIGS. 2 and 4 given a closed valve in order to prevent the formation of trickling. This design also means the presences of considerable dead spaces.